Soldered joint

ABSTRACT

A soldered joint between articles of at least one of the high melting point metals, high melting point metal carbides and high melting point metal oxides, which joint comprises titanium in which at least the external surface layer of the soldered joint consists of titanium nitride. 
     The soldered joint is preferably used for securing electrodes for high-pressure gas discharge lamps to current supply conductors. 
     The soldered joint may be obtained by soldering with titanium in a rare gas atmosphere whereafter the solder point is heated for some time in a nitrogen-containing atmosphere.

This is a division of application Ser. No. 373,943, filed June 27, 1973and now U.S. Pat. No. 3,923,470.

The invention relates to a soldered joint between articles consistingmainly of at least one of the high melting point metals, high meltingpoint metal carbides and high melting point metal oxides, which jointcomprises titanium. Furthermore the invention relates to an electrode tobe used for a high-pressure gas discharge lamp which is secured by meansof such a soldered joint to a current supply conductor, and to ahigh-pressure gas discharge lamp provided with such an electrode.Furthermore the invention relates to a method of manufacturing the saidsoldered joint.

In the following description and Claims high melting point metals areunderstood to mean metals having a melting point of more than 2400° C.High melting point metal carbides and high melting point metal oxidesare understood to mean metal carbides and metal oxides having a meltingpoint of more than 2000° C and exhibiting no noticeable decomposition atthe said temperature of 2000° C. Examples of such materials aretungsten, tantalum, molybdenum, rhenium, osmium, iridium, niobium,tantalum carbide, tungsten carbide and aluminium oxide.

It is known that a joint between tungsten articles can be obtained bymaking a weld. A zone of molten tungsten is then provided between thearticles, which zone forms the joint between the articles after cooling.Such a welded joint is, however, very brittle and weak predominantlybecause recrystallisation of tungsten in and near the weld occurs due tothe very high temperatures which are required for welding.

U.S. Pat. No. 2,993,112 describes a method of welding tungsten articlesin which a quantity of titanium is provided on the welding pointwhereafter the welding point is heated in an inert atmosphere to such atemperature that tungsten near the welding point melts and is mixed withthe molten titanium. Although the welded joint obtained in this manneris not brittle itself, it is found that due to the high temperaturewhich is required to melt tungsten a considerable recrystallisation andcrystal grain growth of tungsten in the vicinity of the welding pointoccurs so that the articles have only a slight mechanical strength.

U.S. Pat. No. 3,365,779 describes a method of soldering articles oftungsten to articles of sintered aluminium oxide. A quantity of titaniumis then provided on the solder point between the articles whereafter thesolder point is heated to a maximum temperature of approximately 1800° Cin an inert atmosphere. Such a soldered joint has a satisfactorymechanical strength. It has, however, the drawback that it is notresistant to temperature approximating the melting point of titanium(approximately 1800° C). Furthermore it is found that the titaniumsolder evaporates to a considerable extent at temperatures which areconsiderably lower than the melting point of titanium.

In many cases it is desirable to have a soldered joint between articlesof high melting point material which can be brought to a temperature of,for example, 1400°-1600° C without their becoming inadmissibly weak andwithout leading to a large evaporation of the soldering material. Such ajoint might be used, for example, very advantageously for securing anelectrode for a high-pressure gas discharge lamp to a current supplyconductor.

For securing a tungsten electrode to a tungsten current supply conductorit is known to use a low melting point solder, for example, platinum.The use of platinum has, however, the drawback that the electrode duringoperation of the lamp easily comes loose, because the soldered joint mayreach a temperature of, for example, 1400°-1500° C. in the lamp.Furthermore the vapour pressure of platinum at these temperatures isinadmissibly high. It is also known to use a high melting point solder,for example, molybdenum having a favourable vapour pressure for thejoint between electrode and current supply conductor. The melting pointof molybdenum is, however, very high (approximately 2600° C.) so thatthe required high soldering temperature results in a strongrecrystallisation of current supply conductor and/or electrode aroundthe solder point so that a weak electrode construction is obtained.

The object of the invention is to provide a soldered joint between highmelting point materials which does not have the above-describeddrawbacks of the known soldered joints and which can be brought to veryhigh temperatures without any objection.

According to the invention a soldered joint between articles whichconsist mainly of at least one of the high melting point metals, highmelting point metal carbides and high melting point metal oxides, whichjoint comprises titanium is characterized in that at least the externalsurface layer of the soldered joint consists of titanium nitride.

A soldered joint according to the invention has a very high mechanicalstrength. This strength is maintained even at high temperatures. It hasbeen found that this soldered joint does not come loose even attemperatures which are hundreds of degrees higher than the melting pointof titanium (approximately 1800° C.). The titanium nitride layer may berelatively thin. The titanium nitride layer in a soldered jointaccording to the invention has the additional advantage that it providesan eminent protection against the release of titanium vapour. The vapourpressure of titanium nitride itself (melting point approximately 2950°C.) is very low. An important advantage of a soldered joint according tothe invention is that it can be obtained at a comparatively lowtemperature so that the mechanical properties of the articles to besoldered are substantially not affected.

The thickness of the titanium nitride layer in a soldered jointaccording to the invention may be chosen within very wide limits.However, titanium nitride layers whose thickness is at least 1 μ arepreferred. In fact, it is then ensured that there is a sufficientstrength at comparatively high temperatures and a satisfactoryprotection of the titanium metal in the soldered joint. The solderedjoint may consist entirely of titanium nitride.

A very advantageous embodiment according to the invention is anelectrode intended for a high-pressure gas discharge lamp whichelectrode consists mainly of at least one of the metals tungsten andtantalum and is secured by means of a soldered joint according to theinvention to a current supply conductor consisting mainly of at leastone of the metals tungsten, molybdenum and tantalum. The tungsten and/ortantalum of the electrode material may be mixed or alloyed with, forexample, thorium oxide, rhenium or tantalum carbide. Such an electrodeaccording to the invention may be introduced into a lamp at a hightemperature without the drawbacks of loosening or of the formation of anunwanted metal vapour.

Such an electrode according to the invention is preferred, whichconsists of a mainly cylindrical body having a conical or sphericaltermination for the discharge, in which the end face of the electroderemote from the termination is provided with a hole in which the currentsupply conductor is secured by means of titanium solder and in which theexternal surface layer of the solder is converted into titanium nitride.Such an electrode construction has the advantage that it has a very highmechanical strength. For soldering the current supply conductor to theelectrode a temperature which is not much higher than the melting pointof titanium may be sufficient. Recrystallisation and consequentlybrittleness of electrode and current supply conductor is thensubstantially prevented.

An electrode according to the invention is preferably used in a shortarc xenon discharge lamp. In such a lamp the electrode construction issubjected to very high thermal and mechanical load. The electrode in thelamp may be an anode or a cathode or, in the case of an AC operatedlamp, alternately an anode and a cathode.

The soldered joint, particularly the soldered joint between a currentsupply conductor and an electrode is preferably manufactured by means ofa method according to the invention which is characterized in that aquantity of titanium is provided in the vicinity of the solder pointbetween the articles consisting mainly of high melting point metal, highmelting point metal carbide and/or high melting point metal oxide, thesolder point being heated in a rare gas atmosphere at a temperaturewhich is higher than the melting point of titanium, the solder pointbeing subsequently heated in a nitrogen-containing atmosphere andfinally being cooled.

In a method according to the invention a soldered joint is firstly madewith titanium. The solder point is then provided with titanium and issubsequently heated to a temperature which is higher than the meltingpoint of titanium. This heat treatment is to be effected in a rare gasatmosphere so as to avoid contamination of the solder point andpremature formation of titanium nitride at the solder point. Solderingwith titanium may be performed, for example, by heating the solder pointwith a high-frequency coil in the rare gas atmosphere until titaniumflows. Then a temperature which is not much higher than the meltingpoint of titanium is required for only a short period. After themanufacture of the titanium weld, which has a grey-blue appearance aftercooling, the solder point is to be heated in a nitrogen-containingatmosphere. It is advantageous to change over from the heat treatment ina rare gas without any interruption and hence without intermediatecooling to the heat treatment in a nitrogen-containing atmosphere.During the second heat treatment the titanium is superficially nitrated.The thickness of the titanium nitride film then formed is dependent onthe temperature and on the nitrogen concentration during nitration andfurthermore on the duration of nitration. After cooling, which may beeffected, for example, in air, in nitrogen, in an inert or in a reducingatmosphere, the soldered joint having a gold-yellow or bronze colouredappearance is ready.

In a method according to the invention argon is preferably used as arare gas because this gas is cheap and can be obtained in a very pureform.

In a method according to the invention the heat treatment in anitrogen-containing atmosphere is preferably effected at a temperatureof between 1600° and 1900° C. for at least 1 second. In fact, theformation of a suitable titanium nitride layer is then ensured. Ifnitration is effected at temperatures of less than 1600° C. the timerequired for the formation of a nitride film having a desired thicknessfor practical uses is too long; temperatures of more than 1900° C. areless desirable because then the mechanical properties of the articles tobe soldered may be affected, for example, by recrystallisation.

In a method according to the invention nitrogen or a mixture of nitrogenand hydrogen is preferably chosen as the nitrogen-containing atmospherebecause optimum titanium nitride layers can thus be obtained.

The invention will now be further described with reference to some testsand a drawing.

In the drawing,

FIG. 1 is a cross-section of an embodiment of a soldered joint accordingto the invention and

FIG. 2 diagrammatically shows the cross-section of an electrodeaccording to the invention which is suitable for a short-arc xenondischarge lamp.

TEST 1

One end of a tungsten rod having a diameter of 1.7 mms is placed againstthe end of a tungsten rod having a diameter of 2.5 mms and a thin foilof titanium is provided on the solder point between the ends of the rodsfacing each other. The solder point is heated by means of ahigh-frequency furnace while passing through a stream of argon (2 l perminute) until titanium flows. Then a temperature of approximately 1850°C. is reached. The gas stream through the furnace is then replaced by anitrogen stream (3 l per minute) at which the temperature is maintainedconstant at 1850° C. After 8 seconds the furnace is switched off so thatthe solder point cools. During cooling a stream (4.5 l per min.) ofuninflammable mixed gas (a mixture of nitrogen and hydrogen) is passedinto the furnace. The soldered joint thus obtained is shown in across-section in FIG. 1 of the drawing. This Figure shows a sketch madefrom a photograph (enlargement approximately 100 ×) of a schliff of thesoldered joint. In FIG. 1, 1 is the tungsten rod having a diameter of1.7 mms and 2 is the tungsten rod having a diameter of 2.5 mms. 3 is thetitanium solder. The titanium nitride film 4 formed is clearly visibleand has a thickness varying between approximately 33 and 62 μ. It isclearly shown that the film 4 constitutes a satisfactorily uninterruptedlayer completely covering the titanium solder.

Analogously as described above, a large number of tests was performedduring which it was found that the thickness of the nitride film can bechosen within wide limits by suitable choice of the circumstances duringnitration (temperature, time, nitrogen concentration).

TEST 2

Two tungsten rods (diameter 1.6 mms) are soldered together at theirends. For this purpose the rods are provided with a quantity of titaniumon the solder point and subsequently heated in an argon stream up to atemperature of approximately 1850° C. Subsequently the rods aremaintained for 6 seconds at this temperature in a nitrogen stream andthen cooled off. Measurements on the tensile stength (2 tests) yieldedthe values 100 and 124 kg.

For the purpose of comparison the tensile strength was measured on thejoint between two similar tungsten rods which had been soldered in thesame manner as described above with titanium but in which the heattreatment in nitrogen had not been used. The tensile strength (2 tests)in this case was 112 and 114 kg. A tungsten rod having a diameter of 1.6mms which was subjected to a heat treatment up to approximately 1850° C.(analogous to the above-mentioned heat treatment during soldering) isfound to have a tensile strength of 148 kg.

TEST 3

The electrode shown in FIG. 2 has a cylindrical electrode body 1 ofthoriated tungsten (tungsten with 1.5 % by weight of ThO₂) having adiameter of 8 mms. The electrode has a conical tip 2 which serves as atermination for the discharge. The end 3 of the electrode body isprovided with an axially located hole 4 having a diameter of 3.5 mms. Atungsten current supply conductor 5 having a diameter of 2.5 mms issecured in the hole 4 by soldering. The soldered joint is obtained byintroducing approximately 25 mgs of titanium in the hole 4 and byheating the electrode with its tip down and provided with the currentsupply conductor 5 in a high-frequency furnace in an argon stream untilthe titanium melts. Subsequently nitrogen is passed through the furnacefor 10 seconds at a temperature of approximately 1850° C. A titaniumnitride film 7 having an average thickness in the order of 50 μ is thenproduced on the titanium solder 6.

In order to investigate the temperature resistance of the electrodeconstruction thus obtained, which is suitable for a short-arc xenondischarge lamp, the construction is suspended vertically in a furnacefrom the current supply conductor with the electrode body pointingdownwards and subsequently heated under argon to approximately 2500° to2600° C. It was found that the electrode body did not get loose from thecurrent supply conductor.

Completely analogously as described in the above-mentioned testsrelating to soldered joints of articles mainly consisting of tungsten,soldered joints may be manufactured of articles consisting of the othermentioned high melting point materials.

What is claimed is:
 1. An electrode for use in high-pressure gasdischarge lamp, said electrode consisting mainly of at least one metalselected from the group consisting of tungsten and tantalum and securedby soldered joint means to a current supply conductor consisting mainlyof at least one metal selected from the group consisting of tungsten,molybdenum and tantalum, said soldered joint means comprising titaniumand having at least an external surface layer consisting of titaniumnitride.
 2. An electrode as claimed in claim 1, consisting of a mainlycylindrical body having a conically or spherically shaped terminationfor the discharge, the end face of the electrode remote from thetermination being provided with a hole in which the current supplyconductor is secured by means of titanium solder, characterized in thatthe external surface layer of the solder is converted into titaniumnitride.
 3. An electrode as claimed in claim 2 intended for a short-arcxenon discharge lamp.
 4. A high-pressure gas discharge lamp providedwith an electrode as claimed in claim
 1. 5. A method of soldering anelectrode to a current supply conductor as claimed in claim 1 comprisingheating a solder point between said electrode and current supplyconductor in the presence of titanium in a rare earth atmosphere at atemperature above the melting point of titanium then heating theresultant titanium containing solder joint in nitrogen and then coolingsaid solder joint.
 6. A method as claimed in claim 5, characterized inthat argon is used as a rare gas.
 7. A method as claimed in claim 5,characterized in that the heat treatment in a nitrogen-containingatmosphere is effected at a temperature of between 1600° and 1900° C.for at least 1 second.
 8. A method as claimed in claim 5, characterizedin that the nitrogen-containing atmosphere consists of nitrogen or amixture of nitrogen and hydrogen.